Last
time, we left off with echinoids (sea urchins) and there is much left
to explore about them. In fact, we have barely scratched the
surface. Recently I saw a scanning electron photomicrograph in a
Zeiss catalog and it was an image of a tiny section of sea urchin
test and the degree of fenestration was staggering. Swiss Cheese–eat
your heart out!

However,
before we get to tests and the detailed structure of other
structures, I want to show you a few more examples of complete dried
urchins including one that might be called the “king of the
echinoids”–Heterocentrotus mammilatus–a “slate pencil” urchin with thick spines (and remember that these are
essentially chalk), which was used in that very way–as slate
pencils in coastal school rooms. It’s real, it works, I’ve
tried it.

These
very impressive spines, by the way, have an almost triangular cross
section.

My
personal advice regarding making such sections: DO NOT TRY THIS AT
HOME! It is a documented fact that 97.3% of microscopists who tried
to produce thin sections of sea urchin spines went insane. You might
have to take that figure cum
grano salis since
all philosophers are liars and I am a philosopher, therefore I am
telling the truth. Or consider Mark Twain’s remark (which was a
popularization of Benjamin Disraeli’s phrase): “There are
three kinds of lies: lies, damned lies, and statistics.”I’ve
always loved to indulge in making up statistics–people are so
deliciously gullible. However, in this case, there is no question
that trying to produce such sections is enormously frustrating and
requires saintly patience unless you either have an indentured lab
assistant who enjoys drudge work or some very high-tech equipment.

It
is astonishing how many extraordinarily fine slides of cross sections
of sea urchin spines were produced in the last half of the 19th
and the early 20th
Century which might explain the great increases in the population of
madhouses in England and Germany during that period.

My
few attempts ended in broken bits usually just when I thought I was
reaching a point where I had a usable thin section. So, now I have
come to a compromise when I want to look at the internal structure of
spines; I break them into very small pieces, select out the most
promising, polish them a little with an emery board, give them a
quick bath in diluted acid, rinse and examine. Below is an example
of such a preparation of a spine of Strongylocentrotus
purpuratus.

For
truly fine thin sections, I now rely on eBay and occasionally I even
come across a few I can afford and manage to snag them and I’d
like to share them with you–by way of images, that is,–you’re
NOT getting the slides.

While
we’re looking at neat pictures, let’s look at some sea
urchin tests since we’ve been talking about them off and on.

The
tests pose many puzzles, posers, and conundrums like the spines. How
are they constructed? Are there specialized cells involved in the
deposition of the calcareous crystals? Clearly, there must be
different sets of genetic coding between species to produce such
radically different tests.

Let’s
start by taking a closeup look at a small section of the test of a
Colobocentrotus.
The first image shows 5 small, flat, perforated plates attached to a
larger plate.

I
don’t know why but, for some reason, they remind me of the foot
pedals of a pipe organ.

I’ll
show you a closeup of the smaller plates. Here we get a clear
indication of their crystalline structure from the way the light
glistens off the surface.

To
give you an idea of what an incredible engineering project a sea
urchin is I’ll show you a picture of the bottom of a test which
I cut open–another act of Rampant Vandalism just to satisfy
your insatiable curiosity (and mine own, of course). It is justified
by the fact that it opens up a whole new set of perspectives.

In
the center, we can see the altar to Poseidon where all the cells come
to worship. Just kidding. Actually, it’s much more
interesting than that. That structure is known as Aristotle’s
Lantern and the ancient Greek philosopher was the first to describe
it over 2,000 years ago. Like you, Aristotle was possessed by
boundless curiosity and wrote extensively on Natural History.

And
here are 2 images of a lantern which has been isolated. It is from
an urchin of the genus Toxopneustes.

This
apparatus consists of five teeth and a series of plates, supports,
and cross bars which comprise the lantern–usually about 40
pieces. Below are 2 images of the pieces of a disarticulated lantern
from Toxopneustes.

Next,
I’ll show you a tooth and 3 major types of components of a
lantern from Strongylocentrotus
purpuratus.
First, one of the teeth.

There
are 4 other usual types of structures: pyramids, epiphyses, auricles,
and rotules. Here are examples of 3 of them.

These
can be arranged on a piece of stiff black card putting the matching
structures in rows. This makes an informative and attractive
display.

In
the first image of the lantern, you can get a sense for how it
extends through the test and the ends of the teeth are visible from
the other sides. Going back to the original image of the lantern on
the half shell, we can also see part of the rows of perforations
through which the tube feet extend.

A
few spines almost always break off under the rigors of being shipped
and I always save them and set them aside. Later, I sort through
them and select a suitable variety and mount them in a Riker box.

Spines
vary widely in size and construction.

Here
are some of the tiny spines on the test of a
Mespilia,
that little “Christmas” urchin that we looked at in Part
1. Note that they are banded with alternating pale green and red. The
reason for such coloration is unknown.

Coelopleurus
tends
to have long, bowed spines that are 4 or 5 times the length of the
diameter of the test and even more remarkable has tube feet that can
extend out even beyond the tips of the spines.

Strongylocentrotus
franciscanus,
along with a number of other species, has a fascinating arrangement of
crystals along the outer surface of the spines. The crystals are
arranged in such a way that their tips all point to the top end of
the spine. As a consequence, if you slide your thumb and finger from
the base to the tip, your fingers move smoothly. However, if you
reverse direction, you will find yourself unable to move down the
spine due to the tips of the crystals impeding such motion. The
outer part of the spines is like a long stack of tiny crowns
gradually diminishing in size as they near the tip.

Some
members of the genus Diadema
have
spines slightly over a foot long which have poison glands near the
tip, and which can, when triggered, produce a toxin which is
excruciatingly painful to humans who are unfortunate enough to have a
close encounter of the spine kind.

If
we look closely at a portion of a spine from Strongylocentrotus
purpuratus,
we can learn more about the structure of the spine. There are very
thin lines of crystals running from the base up toward the terminus,
but even more interesting is the fact that the spine consists of a
series of thick disks piled on one another rather like the
construction of a Greek column.

If
we take a tiny piece of this spine and crush it, we can get a glimpse
into the way the crystals are aligned.

Habitat
can play an important role in terms of the characteristics of spines
in some genera. Colobocentrotus,
for example, tends to reside in coastal areas where there is heavy
surf action. As a consequence, the spines are greatly flattened
which prevents the urchin from being easily dislodged by the pounding
surf. This modification is a significant one and shows up
dramatically.

Another
attractive and highly interesting echinoid is Stereocidaris
granularis var. rubra.
Notice the incredible number of “hitchhikers” which have
attached to the spines.

I
have been haunted by those bizarre spines of Psychocidaris
and
so I have decided to soon return to them and break 1 or 2 apart to
see what I can discover. I’ll try to remember to let you know
what I find. In the meantime, here is a close up of a single spine
which looks rather like an ancient urn pulled out of an
archaeological dig.

This
essay is getting a bit long, so some other wonders such as
pedicellaria and sphaeridia, along with the whole range of irregular
urchins, will have to wait until Part 3.

However,
just to show you how much fun irregular urchins can be, let me show
you an image which with its 2 “eyes” and “extended
tongue” could be an African mask of comedy.